Epidemiology of preterm birth and neonatal outcome

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Abstract

In industrialized countries, 5–11% of infants are born preterm (<37 weeks' gestation), and the rate has been increasing since the early 1980s. Preterm births account for 70% of neonatal deaths and up to 75% of neonatal morbidity, and contribute to long-term neurocognitive deficits, pulmonary dysfunction and ophthalmologic disorders. In the past several decades, major progress has been made in improving the survival of extremely premature newborns, mostly attributable to timely access to effective interventions that ameliorate prematurity-associated mortality and morbidity such as antenatal administration of corticosteroids and exogenous surfactant therapy, rather than preventing preterm births. However, the societal and healthcare costs to care for survivors with severe morbidity and neurological handicaps remain substantial. Future research should concentrate on the ways to reduce long-term health sequelae and developmental handicaps among survivors of infants born preterm, as well as elucidating the mechanisms and aetiology of preterm births.

Introduction

The World Health Organization defines preterm birth as the delivery of an infant between 20 and 37 weeks' gestation. The preterm delivery rate has been reported as 11% in the USA, between 5% and 7% in Europe,1 and approximately 6.5% in Canada.2 Preterm birth is a major public health concern. In industrialized countries, preterm birth is responsible for 70% of neonatal mortality and 75% of neonatal morbidity,3 and contributes to long-term neurodevelopmental problems, pulmonary dysfunction and visual impairment.4 The provision of intensive care for preterm newborns is an enormous burden on the healthcare system. For example, the cost of caring for preterm babies in the USA has been estimated at $8 billion annually.3

Spontaneous preterm labour of unknown aetiology accounts for 40–50% of all preterm deliveries, with the remainder resulting from such obstetrical complications as premature rupture of membranes (PROM) (25–40%) and obstetrically indicated preterm delivery (20–25%).5 As the increased risk of neonatal mortality and morbidity in early preterm births (<32 weeks' gestation) is much higher than those born between 32 and 37 weeks' gestation, more attention has been paid to the former subgroup.6 However, although the relative risk in those born at 32–37 weeks' gestation is much lower, these births are much more common than those at less than 32 gestational weeks; therefore, the public health impact of this group is still high.7

Section snippets

Risk factors of preterm births

Epidemiological studies have identified several risk factors for preterm birth such as prior preterm birth,8 black race,9 teenage or older mothers,10 those with low education and of low socio-economic status,10 cigarette smoking,8 unmarried or not living with a partner,8 heavy and/or stressful occupation,11 low maternal prepregnancy body mass index,8 and poor or excessive weight gain.12 Medical and obstetrical complications including multifetal pregnancy,13 gestational or pre-existing diabetes,

Gestational age

Magowan et al.20 observed that neonatal mortality fell rapidly with advancing gestation, from 795/1000 live births at 24 weeks' gestation to 9/1000 live births at 36 weeks' gestation. Wigton et al.21 found that respiratory distress syndrome (RDS), grade III or IV intraventricular haemorrhage (IVH) and necrotizing enterocolitis (NEC) were relatively frequent at ≤33 weeks' gestation (19.4%, 8.1% and 4.8%, respectively) but that the rates were all less than 2% in the 151 neonates born at ≥34

Secular trends of preterm births

For unknown reasons, the incidence of preterm birth has been increasing in many industrialized countries since the early 1980s.9, 30, 31, 32 It seems that this increase has been more evident among white and wealthy populations. Demissie et al.9 found that among whites, preterm births increased from 8.8% of live births in 1989 to 10.2% in 1997, a relative increase of 15.6%, whereas among blacks, preterm births decreased by 7.6% (from 19% to 17.5%) during the same period. Craig et al.30 found

Neonatal mortality

Due to major advances in medical technology and wide implementation of regionalization of perinatal care, the survival of extremely premature newborns in industrialized countries has improved dramatically over the past three decades35 but has increased since the mid 1990s. Joseph et al.36 compared infant mortality rates in Canada between 1985–1987 and 1992–1994, after grouping the study subjects into specific gestational age and birthweight categories. They found that the magnitude of the

Prevention and management of preterm birth

The focus for the prevention and management of preterm birth should be on reducing prematurity-related neonatal mortality and morbidity, because preterm birth is important as it causes an increased risk of neonatal mortality and morbidity. On the other hand, full-term infants may die or have diseases because of events in late gestation or during labour and delivery. As a result, delaying birth to term is not the sole goal of prevention and management of preterm birth. Indeed, under certain

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